In the applications of FOLs, it is very useful to isolate failures to a particular transmitter or receiver and to control the optical output power of the transmitter in the presence of environmental changes or transmitter aging. To do this, the optical power output of the transmitter must be monitored. This monitoring is best done without an optical power budget penalty. In prior art devices, an optical power splitter is inserted into the optical path in order to isolate faults. The addition of this component reduces the power budget available for the optical path since it has a significant power loss.
The invention accomplishes output power monitoring without the need for additional components such as the power splitter of the prior art. The invention utilizes a feature of wavelength division multiplexers (WDMs) typically already included in FOLs. Wavelength division multiplexers are used in fiber optic links to allow full duplex operation over single fibers. A functional diagram of a WDM is shown in FIG. 1. Within the WDM, a spectrally selective mirror routes light of different wavelengths to different outputs. During this routing process, a small portion of the transmitter output is reflected off of the mirror as shown by the light ray marked C. This reflected light contains about 10% of the transmitted energy and occurs as a natural consequence of characteristics inherent in the construction of a spectrally selective mirror. In known fiber optic systems such reflections generally result in wasted light energy. The invention provides a means for utilizing the reflected energy which would otherwise be discarded. In this way the invention does not affect the power budget of a conventional WDM optical system while providing additional measurement and control capabilities.
Using the principles of the invention, a full duplex fiber optic transmission system is designed incorporating two wavelength division multiplexers. The WDMs are designed to route the reflected light from the transmitter to an optical detector.